1. Field of the Invention
The present invention relates to a stacked electric double layer capacitor comprising a plurality of capacitor elements which are stacked in series with each other. More particularly, the present invention relates to a structure for storing capacitor elements in a stacked electric double layer capacitor.
2. Description of the Background Art
Due to recent development of semiconductor techniques, not only industrial apparatuses but consumer apparatuses generally have CPUs on board. These apparatuses are usually provided with a RAM for storing programs and data. It is therefore necessary to regularly apply a voltage of at least about 2 V to the RAM, in order to hold the memory.
Such a voltage for memory protection can be supplied by a battery or an electric double layer capacitor. The electric double layer capacitor is mainly employed as a backup power source in recent years in consideration of excellent maintainability, facility, degree of freedom in design, and the like.
A semiconductor device such as a CPU or a RAM is increasingly reduced in operating voltage, so that the device can be advantageously applied to a portable apparatus. To this end, there is a need for miniaturization and reduction in thickness of the electric double layer capacitor used as the backup power source.
Such an electric double layer capacitor indispensably requires an electrolyte, which is generally prepared from a nonaqueous solvent type organic electrolyte or an aqueous electrolyte. The difference between these electrolytes resides in decomposition voltages, so that organic electrolyte is generally suitable for miniaturizing and reducing the thickness of the capacitor made using it since the organic electrolyte can easily attain a higher voltage per unit cell as compared with the aqueous electrolyte.
FIG. 6 shows a general unit structure of a conventional electric double layer capacitor 1 employing an organic electrolyte. Referring to FIG. 6, the electric double layer capacitor 1 comprises polarizable electrodes 2 and 3, made of a carbonaceous material such as activated carbon, which are opposed to each other through a separator 4 to form an element 5. Such an element 5 is impregnated with an electrolyte and held between first and second case halves 6 and 7, to be stored in a case defined by the case halves 6 and 7. The first and second case halves 6 and 7, which also serve as external terminal means, are electrically insulated from each other by an insulating gasket 8.
The unit cell of such an electric double layer capacitor 1 employing an organic electrolyte exhibits a withstand voltage of about 1.8 to 2.8 V.
In conformity to the required rated voltage, therefore, a required number of such unit cells are connected in series with each other. In general, a plurality of such electric double layer capacitors 1, as shown in FIG. 6, are stacked to be integrated with each other, in order to attain such series connection of the unit cells. Under the present circumstances, the voltage of an ordinary semiconductor device is 5 V. In general, therefore, two such electric double layer capacitors 1 are stacked to be set at a rated voltage of 5.5 V.
On the other hand, Japanese Patent Laid Open Application 63-187613 (1988) discloses an integrated two-cell stacked electric double layer capacitor which comprises a single metal armoring case and two pairs of capacitance generators stored therein in a series-connected state.
FIG. 7 shows an example of an electric double layer capacitor 9 proposed in the above literature. Referring to FIG. 7, the electric double layer capacitor 9 comprises capacitor elements 10 and 11 which are impregnated with an electrolyte. The capacitor element 10 includes polarizable electrodes 13 and 14 which are opposed to each other through a separator 12, while the other capacitor element 11 includes polarizable electrodes 16 and 17 which are opposed to each other through another separator 15. The capacitor element 10 is held between a first electrode plate 18, which serves both as external terminal means and an armoring case, and a second electrode plate 19. The second electrode plate 19 is arranged between the capacitor elements 10 and 11. The capacitor element 11 is held between the second electrode plate 19 and a third electrode plate 20, the third electrode plate 20 serving both as an external terminal means and an armoring case. Respective peripheral portions 21, 22 and 23 of the first, second and third electrode plates 18, 19 and 20 are electrically insulated from and integrated with each other by insulating gaskets 24 and 25.
According to the structure shown in FIG. 7, it is possible to manufacture an integrated two-cell stacked electric double layer capacitor 9 corresponding to a rated voltage of 5.5 V, through about the same number of steps as those for assembling the electric double layer capacitor 1 defining the unit cell shown in FIG. 6.
With recent miniaturization and reduction in thickness of electronic apparatus, also desired are miniaturization and reduction in thickness of a backup power source provided in the electrode apparatus. In particular, it is desirable that the thickness of such a backup power source be not more than the armoring height of a general semiconductor device.
In the structure shown in FIG. 6, it is possible to somewhat reduce the thickness of the electric double layer capacitor 1 by reducing the insulating gasket 8 in thickness. However, the electric double layer capacitor 1 cannot be sufficiently reduced in thickness since it is impossible to excessively reduce the thickness of the insulating gasket 8 in consideration of sealability and strength of the device. Reduction in thickness of the device is restricted also since it is necessary to stack two or more electric double layer capacitors 1 in order to conform the same to the general rated voltage of 5.5 V.
In the structure shown in FIG. 7, on the other hand, the two capacitor elements 10 and 11 are previously stacked and integrated with each other. However, it is difficult to reduce the thickness of this structure since the insulating gaskets 24 and 25 are employed as sealing means similar to the structure shown in FIG. 6. Moreover, the sealing structure in FIG. 7 is more complicated as compared with that shown in FIG. 6.
In the structures shown in FIGS. 6 and 7, the insulating gaskets 8, 24 and 25 are made of resin, and are held between the upper and lower case peripheral portions under pressure to attain sealing. Thus, the electrolytes may tend to leak due to stress relaxation of the resin, or in similar conditions.